Author Topic: Vacuum Tubes Overview  (Read 14053 times)

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Offline vacuumtubes

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Vacuum Tubes Overview
« on: Nov 29, 2001 at 08:57 PM »
BASIC:
The Vacuum Tube is an electronic component that is made from an evacuated tube from where it derived its name. There are many types developed before the rise of the solid-state revolution. Most of these have been replaced by solid-state versions.
There seems to be a growing interest in one old analog technology that just would not stay behind: VACUUM TUBE AUDIO AMPLIFIERS.
At first, one may even tempted to think that this is just a retro fad. And the debate on the Subjective Issues whether a Solid-State amp is better than a Vacuum Tube one will most likely not die down in the near future.
Of course, we need not join the Battle. Instead, we can be a little objective by taking a closer look at Vacuum Tubes and see the the characteristics that made them quite indispensable to many Audiophiles and most especially to electric instrument users.
Since the rise of solid-state electronics, no electronics school would be caught teaching old technology. So for most of us too young to know, all we ever got to see are cathode ray (CRT’s = picture tube) and microwave tubes. Don’t laugh, these are legit vacuum tubes, although not for audio.
Structurally Vacuum Tubes may be classified according to the number of electrodes like diodes, triodes, pentodes, and others.


HISTORY:
1904 - British scientist John Ambrose Fleming first showed his device
to convert an alternating current signal into direct current. The "Fleming diode" was based on an effect that Thomas Edison had first discovered in 1880, and had not put to useful work at the time. Many other inventors tried to improve the Fleming diode, most without success.

1907 - The only one who succeeded was New York inventor Lee de Forest. He patented a bulb with the same contents as the Fleming diode, except for an added electrode.

Between 1907 and 1960 - a staggering array of different tube families was developed, most derived from de Forest's invention. The vast majority of these tubes have been replaced by semiconductors, leaving only a few types in regular manufacture and use--mostly in specialized audio applications, or in high-power radio equipment.

Audion, was the first successful electronic amplifier. It was the genesis of today's huge electronics industry.

A typical modern vacuum tube is a glass bulb with wires passing through its bottom, and connecting to the various electrodes inside. Before the bulb is sealed, a powerful vacuum pump sucks all the air and gases out. This requires special pumps which can make very "hard" vacuums. To make a good tube, the pump must make a vacuum with no more than a millionth of the air pressure at sea level (one microTorr, in official technical jargon), and preferably less. The "harder" the vacuum, the better the tube will work and
the longer it will last. Making an extremely hard vacuum in a tube is a lengthy process, so most modern tubes compromise at a level of vacuum that is adequate for the tube's application--pumping is done for less than one half-hour. This is a major reason why NOS tubes are superior to modern production, in the old days tubes were simply pumped longer, because production costs were less.

New old stock (NOS): a term borrowed from antique collectors to describe a tube or other component no longer being manufactured and found unused in its original factory packaging.

The Only Reliable Way To Determine The Health Of A Tube Is To Test It ELECTRICALLY. ;)

References:
Electronic Enthusiast, IEEE, National, RCA, Sovtek, Svetlana

vax
11-29-2001 12:25pm

Offline utoy

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Re: Vacuum Tubes Overview
« Reply #1 on: Nov 30, 2001 at 12:25 AM »
I failed ELCE 313.  ;D ;D
« Last Edit: Jan 01, 1970 at 08:00 AM by 1016344800 »

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Advantages Of Vacuum Tubes!
« Reply #2 on: Nov 30, 2001 at 01:12 AM »
Advantages:

1.Highly linear without negative feedback, especially some small-signal types.

2.Clipping is smooth, which is widely considered more musical than transistors.

3.Tolerant of overloads and voltage spikes.

4.Characteristic’s highly independent of temperature, greatly simplifies biasing.

5.Wider dynamic range than typical transistor circuits, thanks to higher operating voltages.

6.Device capacitances vary only slightly with signal voltages.

7.Capacitive coupling can be done with low-value, high-quality film capacitors.

8.Circuit designs tend to be simpler than semiconductor equivalents.

9.Operation is usually in Class A or AB, which minimizes crossover distortion.

10.Output transformer in power amp protects speaker from tube failure.

11.Maintenance tends to be easier because tubes can be replaced by user.

Courtesy of IEEE Spectrum 1998 :)

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Disadvantages of Vacuum Tubes!
« Reply #3 on: Nov 30, 2001 at 01:18 AM »
Disadvantages:

1.Bulky, hence less suitable for portable products.

2.High operating voltages required.

3.High power consumption; needs heater supply.

4.Generate lots of waste heat.

5.Lower power efficiency than transistors in small-
signal circuits.

6.Low-cost glass tubes are physically fragile.

7.More prone to microphonics than semiconductors, especially in low-level stages.

8.Cathode electron-emitting materials are used up in operation, resulting in short lifetimes (typically 1-5 years for power tubes).

9.High-impedance devices that usually need a matching transformer for low-impedance loads, like speakers.

10.Usually higher cost than equivalent transistors.

Courtesy of IEEE Spectrum 1998

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Advantages Of Transistors!
« Reply #4 on: Nov 30, 2001 at 01:59 AM »
Advantages:

1. Usually lower cost than tubes, especially in small-signal circuits.

2. Smaller than equivalent tubes.

3. Can be combined in one die to make integrated circuit.

4. Lower power consumption than equivalent tubes, especially in small-signal circuits.

5. Less waste heat than equivalent tubes.

6. Can operate on low-wattage supplies, greater safety, lower component costs, smaller clearances.

7. Matching transformers not required for low-impedance loads.

8. Usually more physical ruggedness than tubes (depends on chassis construction).

Courtesy of IEEE Spectrum 1998

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Disadvantages Of Transistors!
« Reply #5 on: Nov 30, 2001 at 02:03 AM »
Disadvantages Of Transistor:

1. Tendency toward higher distortion than equivalent tubes.

2. Complex circuits and considerable negative feedback required for low distortion.

3. Sharp clipping, in a manner widely considered non-musical, due to considerable negative feedback commonly used.

4. Device capacitances tend to vary with applied voltages.

5. Large unit-to-unit variations in key parameters, such as gain and threshold voltage.

6. Stored-charge effects add signal delay, which complicates high frequency and feedback amplifier design.

7. Device parameters vary considerably with temperature, complicating biasing and raising the possibility of thermal runaway.

8. Cooling is less efficient than with tubes, because lower operating temperature is required for reliability.

9. Power MOSFET’s have high input capacitances that vary with voltage.

10. Class B totem-pole circuits are common, which can result in crossover distortion.

11. Less tolerant of overloads and voltage spikes than tubes.

12. Nearly all transistor power amplifiers have directly-coupled outputs and can damage speakers, even with active protection.

13. Capacitive coupling usually requires high-value electrolytic capacitors, which give inferior performance at audio-frequency extremes.

14. Greater tendency to pick-up radio frequency interference, due to rectification by low-voltage diode junctions or slew-rate effects.

15. Maintenance more difficult; devices are not easily replaced by user.

16. Older transistors and IC’s often unavailable after 20 years, making replacement difficult or impossible.

Courtesy of IEEE Spectrum 1998
« Last Edit: Jan 01, 1970 at 08:00 AM by 1016344800 »

Offline DVDiva

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Re: Vacuum Tubes Overview
« Reply #6 on: Nov 30, 2001 at 02:35 AM »
My favorite food is pandesal with kesong puti. Ay sorry, wrong thread!! ;D

Offline vacuumtubes

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Re: Vacuum Tubes Overview
« Reply #7 on: Nov 30, 2001 at 02:45 AM »
okey lang Ms. DVDiva! ikaw pa! ;)

o eto matanong kita, ano ang mas masarap between pandesal and tasty? ;)

and what about kesong puti and the yellow one?

just curious at baka mag-open pa ako ng bagong thread niyan,hmmm. ;D

vax
« Last Edit: Jan 01, 1970 at 08:00 AM by 1016344800 »

Offline alfred

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Re: Vacuum Tubes Overview
« Reply #8 on: Nov 30, 2001 at 03:39 AM »
VAX,

Thanks for the info. Yun pala ang tinatawag na vacuumtubes. ;D ;D ;D

Seriously speaking I think that mostly if not all these weaknesses of transistors are being address by electronics manufacturers when they are use in a circuit. Such that they have provided negative feedback control, close tolerance capacitors, massive heatsinks/fans etc.

Kaya nga binenta ko na yun HK Citation ko baka wala nang mabili na parts. But I wouldn't be caught with an appliance for 20yrs. Except maybe for an airconditioner or a refrigerator.

Airconditioner. Hmmm. ::) Puede mo sigurong start ng thread     ;D

Offline levi

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Re: Vacuum Tubes Overview
« Reply #9 on: Nov 30, 2001 at 04:00 AM »
Ano naman advantages at disadvantages ni Vax?

Offline Compaq

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Re: Vacuum Tubes Overview
« Reply #10 on: Nov 30, 2001 at 04:34 AM »
Quote

Ano naman advantages at disadvantages ni Vax?


*LOL*... hmmmm... let me see ...

Advantages

1. Cool Guy/Kwela ... ;D
2. Nagpapahiram ng Audiophile CDs (at ang gaganda)
3. Very knowledgeable sa stereo setup
4. Magaling kumanta
5. Magaling ding sumayaw  ;D ... isang running man nga bro.. :)
6. Nangungumusta by giving you a call... sa gabi pa... kung kelan ka kumakain at nagpapahinga ;D Seriously, I appreciate that.

DisAdvantages

1.   ;D ;D ;D


This calls for a new thread :)Sino kaya mag-oopen

Cheers!!!
Compaq

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Re: Vacuum Tubes Overview
« Reply #11 on: Dec 07, 2001 at 02:15 AM »
brother COMPAQ,

thank you very much!


brother JT,

dito ko na lang ipopost yung mga topic natin about X10D basta about TUBES!

thanx ;)

vax
« Last Edit: Jan 01, 1970 at 08:00 AM by 1016344800 »

Offline M_Shoe_Maker

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Re: Vacuum Tubes Overview
« Reply #12 on: Dec 07, 2001 at 02:29 AM »
Just called up Upscale Megamall and Park Square.  Ubos na X10D stocks nila.  Dati ang dami nilang stock.  Mukang magiging classic hard to find item to ah.  Baka in the near future, yung mga nag hoard ng X10D, ilabas as NOS with matching premium on the price.
« Last Edit: Jan 01, 1970 at 08:00 AM by 1016344800 »

Offline JT

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Re: Vacuum Tubes Overview
« Reply #13 on: Dec 07, 2001 at 08:20 AM »
Quote

brother JT,

dito ko na lang ipopost yung mga topic natin about X10D basta about TUBES!

thanx ;)

vax


Sorry ha medyo nalihis nga dun sa kabilang thread ... eniway, fully recommended ko lang talaga itong X10-D kaya I can't help but post my reply nung nabasa ko.  

Offline JT

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Re: Vacuum Tubes Overview
« Reply #14 on: Dec 07, 2001 at 08:50 PM »
Quote

hello brother M_Shoe_Maker,

nice discussion about TUBES! :)
my X10D ay JAN PHILIPS ECC88 ang naka-install.

may nagbebenta sa akin dati ng X10D kaya lang mataas na talaga ang price at umaabot ng P10k but brand new DAW. ;)

mahilig ka din pala sa mga tubes.

sayang nga at hindi pa ako nakabili ng mga reserbang tubes before like MULLARD, GE, Westinghouse, Sylvania, Sovtek, or Svetlana.

did you know about TUNGSOL? ???

My father borrowed my X10D and connect it to its DVD515 and Yamaha AVR, the sound is superb!
kaya lang pag sanay ka na na meron ay ayaw mo ng bumalik sa dating set-up at tunog lata na! ;)


Brother Vax,

Mine is using MULLARD CV2493, and the unit the I mention selling here is using JAN PHILIP ECC88 also.  I've heard about TUNGSOL but no idea about it ... ano yun?

« Last Edit: Jan 01, 1970 at 08:00 AM by 1016344800 »

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Re: Vacuum Tubes Overview
« Reply #15 on: Dec 07, 2001 at 09:42 PM »
brother JT,

WOW :o MULLARD!

TUNG-SOL??? ???

click this about the TUNG-SOL VACUUM TUBES HISTORY

http://www.vintagetubeservices.com/page16.html

http://www.vintagetubeservices.com/Default.htm#Music Lover

Six things that make selecting your Tubes an Art:

http://www.vintagetubeservices.com/page4.html

thanx for your interest. ;)

baka pabili ako ng mga replacement Tubes sa iyo. ;)

again Thanx a lot! ;)
« Last Edit: Jan 01, 1970 at 08:00 AM by 1016344800 »

Offline vacuumtubes

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Re: Vacuum Tubes Overview
« Reply #16 on: Dec 07, 2001 at 09:46 PM »
Vacuum Tubes Brands and Learn more about them.

http://www.vintagetubeservices.com/page8.html


Offline JT

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Re: Vacuum Tubes Overview
« Reply #17 on: Dec 07, 2001 at 11:53 PM »

Quote

brother JT,

WOW :o MULLARD!

TUNG-SOL??? ???

click this about the TUNG-SOL VACUUM TUBES HISTORY

http://www.vintagetubeservices.com/page16.html

http://www.vintagetubeservices.com/Default.htm#Music Lover

Six things that make selecting your Tubes an Art:

http://www.vintagetubeservices.com/page4.html

thanx for your interest. ;)

baka pabili ako ng mga replacement Tubes sa iyo. ;)

again Thanx a lot! ;)



K, No problem. Sabihin mo lang kung anong brand at model.
« Last Edit: Jan 01, 1970 at 08:00 AM by 1016344800 »

Offline FASTBACK

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Re: Vacuum Tubes Overview
« Reply #18 on: Mar 08, 2002 at 02:22 AM »
Hey vax very informative itong thread.

Just reading thru the makers I remember most of them kasi when I was still yung my dad has alot of tubes kasi pag nasisira ang TV namin dati siya ang gumagawa, and then he asks his brother who is in the US to buy specific tubes.  Meron din akong nakita na tube tester, kaso di ko na alam kung saan niya nilagay.

Nung nasa High School na ako I have this magazine I forgot the name but it was an audio magazine, they have this particular brand for an amp which they have a solid state model and a tube model same wattage. the difference is the price sobrang mahal nung tube amp.  The brand nga pala niya ay CARVER wow lagi ko ito binabasa kasi na amaze ako sa power capabilities niya.  
« Last Edit: Jan 01, 1970 at 08:00 AM by 1016344800 »

Offline M_Shoe_Maker

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Re: Vacuum Tubes Overview
« Reply #19 on: Mar 08, 2002 at 02:48 AM »
Fastback (VW Type 3),

I think the Carver tube amp you are reffering to is the "Silver Seven" or "Super Seven" or something like that.  If you want to be more amazed, you could look at JADIS monoblocks. WOW!  Wag mo lang tanungin yung price.

Maiba ng konti, I guess you are an air-cooled VW fan.  My fist car was a VW Type 1 Beetle.  I still love those flat four wonders.
« Last Edit: Jan 01, 1970 at 08:00 AM by 1016344800 »

Offline FASTBACK

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Re: Vacuum Tubes Overview
« Reply #20 on: Mar 08, 2002 at 03:11 AM »
As I remember it is a mono tube amp 300w rms, iyun nga yata a "super seven".  Medyo matagtal na kasi nung nabasa ko.

Try to visit the JADIS Monoblocks webpage. thanks

*Yeah a VW Type 3, eversince I rode in my classmates car a VW Type 1 superbeetle na in love na ako sa VW*
« Last Edit: Jan 01, 1970 at 08:00 AM by 1016344800 »

Offline levi

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Re:Vacuum Tubes Overview
« Reply #21 on: Feb 18, 2003 at 02:19 AM »
bump

rtsy

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Re:Vacuum Tubes Overview
« Reply #22 on: Feb 19, 2003 at 10:40 AM »
Lots of Vacuum Tube info and fanatics over at http://www.wiredstate.com/pinoydiophiles

Surf over to the Analog & Tubes section.

Happy listening!

Offline arnoldc

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The Vacuum Tube FAQ
« Reply #23 on: Nov 10, 2003 at 09:16 AM »
I stole this from the 'net, ok?  ;D

Some of you might find this informative and may answer some of your basic questions.

Part 1 of 3
--------------------------------------------------------------------------------

The Vacuum Tube FAQ

by Henry Pasternack

--------------------------------------------------------------------------------
Q: Briefly explain vacuum tube functional elements.
A: The vacuum diode is a two-terminal device having two active elements, the cathode and the anode (or plate). The cathode is designed so that when it is heated, it freely emits electrons. The application of a high positive potential from plate to cathode causes a conventional current to flow through the tube. The magnitude of the current that flows is proportional to the three-halves power of the applied voltage. No current flows when a negative potential is applied. The diode is commonly used as a rectifier in power supply circuits.

The triode is similar to the diode, but contains a third element, located between the cathode and plate, called the control grid. A negative bias applied to the grid relative to the cathode shields the cathode from the electrostatic field due to the plate. Consequently, the flow of current from the plate to the cathode is reduced by the effect of grid. If the negative bias is sufficiently large, no current will flow in the anode circuit and the triode is said to be cut off. Current flow in the triode is thus dependent on both grid and plate voltage.

A second grid, called the screen grid, may be added to the triode. A tube with two grids is called a tetrode. The screen grid shields the control grid from the plate, greatly reducing the dependency of plate current flow on plate voltage. Consequently, the tetrode has a much higher output resistance and behaves more nearly as a current source than the triode. The tetrode is capable of greater voltage amplification than the triode because of its higer output resistance.

A third grid is often added to the vacuum tube, turning it into a pentode. The supressor grid is normally connected to the cathode. Its effect is to reduce the secondary emission of electrons from the plate, allowing the tube to operate over larger excursions of plate voltage. The pentode is thus capable of large output voltages with minimal distortion. Because of this fact, the pentode is a more efficient power amplifier than the triode.

Q: Describe in greater detail the operating characteristics of power vacuum tubes.

A: The vacuum tube is a voltage-controlled device, often compared to the depletion-mode n-channel MOSFET. High plate current will flow when zero control-grid bias is applied; a negative bias is required to control (limit) tube conduction. Compared to a MOSFET, the power tube has very low transconductance, on the order of thousands of micromhos. Maximum continuous cathode current is also low, in the range of several hundred milliamperes. Popular output tubes have plate dissipation ratings on the order of twenty to forty watts.

In order to extract reasonable amounts of power from a low current device, a high load impedance is required. Although it is possible to direct-couple a vacuum tube output stage to a low-impedance loadspeaker load, the vast majority of tube power amplifiers are output transformer-coupled. The output transformer permits the amplifier to operate with reasonable efficiency into eight-Ohm loads.

Popular power tubes are typically operated with DC plate voltages ranging from 350 to 600 volts, screen voltages on the order of 250 to 360 volts, control grid bias voltages from -60 to -30 volts, and idle plate currents of 25 to 75 mA, depending on the application and the tube type.

The output resistance of a triode is moderate, typically no more than a few thousand ohms. Its characteristic curves suggest a voltage-controlled voltage source with a series plate resistance. The tetrode and pentode have output resistances an order of magnitude higher than that of the triode. Because of the greatly reduced dependency of plate current on plate voltage, the tetrode/pentode may be thought of a voltage-controlled current sources. Indeed, the characteristic curves of the pentode are quite similar to those of the MOSFET.

The screen grid significantly reduces the capacitance between the plate and the control grid, reducing Miller feedback and increasing the bandwidth of the tetrode/pentode. The increased output resistance is revealed by a flattening of the characteristic curves. Similar changes occur when two triodes are connected in cascode.

The tetrode/pentode is typically operated with a fixed screen grid potential. Varying the static screen voltage scales the characteristic curves up and down correspondingly. If the screen grid is connected directly to the plate, the tube is said to be operating in "triode" mode and has characteristic curves like those of a conventional triode.

Plate current continues to increase as the control grid potential swings positive above zero volts. At the same time, grid current begins to flow. Many tubes will operate linearly while drawing grid current on signal peaks. Because maximum grid dissipation is low, it is necessary to limit the magnitude and duration of grid current flow. Most amplifiers with high-impedance driver circuits will distort heavily at the onset of grid current.

--------------------------------------------------------------------------------
Henry A. Pasternack
Member Scientific Staff
Bell Northern Research, Montreal
E-mail: [email protected]
« Last Edit: Nov 10, 2003 at 09:22 AM by arnoldc »

Offline arnoldc

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Re:The Vacuum Tube FAQ
« Reply #24 on: Nov 10, 2003 at 09:19 AM »
Part 2of 3

Q: What is "ultralinear" operation?

A: In ultralinear operation, the screen grid is connected to a tap on the output transformer primary such that the screen voltage varies in proportion to the plate signal voltage. The constant of proportion- ality typically ranges from 30-50%, although other ratios will also work. The resulting tube characteristic has properties intermediate between those of the triode and the pentode. In essence, the ultra- linear connection forms a local negative feedback loop around the output stage. This may be advantageous depending on circuit topology and gain distribution. Advocates of ultralinear operation claim this connection combines the best features of triode and pentode mode, while detractors claim it lack the virtues of either.

Q: Compare the general properties of triode, pentode, and ultralinear power amplifiers.

A: It is probably misleading to characterize the various types of output stage connections in terms of sound quality. Many factors contribute to the sound of an amplifier, and a good designer will blend these elements in order to achieve a particular goal. On the other hand, certain sonic qualities are associated with each type of output stage frequently enough that they deserve repeating here. In addition, there are objective differences that are worth mentioning.

The triode amplifier is characterized by low efficiency and low power output. This is because a smaller voltage swing is available from the triode for a given DC plate voltage. Consequently, the triode amplifier burns up more power at idle relative to its peak output. The sound of the triode amplifier is often described as "rich" or "sweet", conveying in a natural and realistic way the harmonic structure of musical instruments and voices.

The pentode amplifier is often described as having a more analytical sound than comparable triode units. Others may accuse it of sounding harsh. Objectively, the pentode output stage tends to produce more high-order distortion products than a comparable triode. In addition, the pentode is more sensitive to load impedance variations and may clip more sharply than the triode.

The ultralinear amplifier combines the benefits (or flaws, depending on your point of view) of the triode and pentode connections. The ultralinear characteristic curves resemble those of the triode in some ways, those of the pentode in others, and have unique characteristics as well (regretably, they are hard to render in ASCII). While the general concensus favors triode mode above all, there seems to be no strong trend supporting ultralinear over pentode mode, or vice-versa. Perusing the high-end magazines, one can find examples of well-regarded amplifiers ueing either type of output connection.

The one certainty is that the ultralinear connection is the cheapest way to get good performance and high power out of a pentode. Whereas a quality pentode design requires a stiffly regulated screen grid supply, all that is needed to implement an ultralinear output stage is a pair of transformer primary taps. Perhaps the economic argument leads the sound quality argument in this case.

Q: Can I convert my amplifier back and forth between pentode, triode and ultralinear modes in order to hear the difference for myself?

A: In general, the answer to this question is "No." Under some circumstances, it may be possible to perform such experiments, but subject to limitations.

If the amplifier is an ultralinear design, it is possible to convert it to pentode operation by connecting the screens to a fixed voltage source. The correct screen voltage depends on the type of output tube, the B+ supply voltage, and the output transformer primary impedance. For audiophile performance, a regulated screen supply may be required. This makes the pentode conversion a major modification.

The most common conversion is to modify an ultralinear or pentode mode amplifier for triode operation. In many cases, this modification can be made successfuly and with little effort, but some caveats apply. One would like to be sure that the maximum triode-connected plate potential is not exceeded. For many EL-34/6L6/KT-66/5881 amplifiers running B+ supplies on the order of 400V, there is no problem converting to triode operation. On the other hand, a 6550 amplifier with 550V on the plates is probably not a candidate for triode conversion without a reduction in B+ voltage. When the conversion is made, a 100 Ohm non-inductive resistor is usually specified, connected directly between the screen and plate pins on the tube socket, to suppress RF instability.

Changing the output stage connection from pentode to triode mode typically lowers the open-loop gain of the amplifier. As a result, the closed-loop global feedback factor also goes down. The output impedance of the amplifier, its sensitivity, the total harmonic distortion and the distortion spectrum will all change. Overload behavior and stability will likely be improved. Typical comments are that the triode-connected amplifier sounds "more relaxed", "warmer", and "sweeter" after the conversion. Whether this is due to an inherent quality of triode-strapped pentodes, or is a consequence of modifying a topology that was not designed with triode output in mind, is open for debate.

--------------------------------------------------------------------------------
Henry A. Pasternack
Member Scientific Staff
Bell Northern Research, Montreal
E-mail: [email protected]
« Last Edit: Nov 10, 2003 at 09:23 AM by arnoldc »

Offline arnoldc

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Re:The Vacuum Tube FAQ
« Reply #25 on: Nov 10, 2003 at 09:21 AM »
Part 3of 3

Q: What about "pure-triode" amplifiers?

A: The vintage triode power tubes, such as the 845, 2A3, and 300B, are classic devices from the earlier days of vacuum tube technology. They are still available in limited supply and at high cost (although there are now Chinese copies on the market that offer a reasonable, lower- price alternative). A significant structural difference between these tubes and more modern units is the use of a directly-heated cathode. In this design, the cathode heater also serves as the emissive element. In contrast, newer tubes employ a separate heater that is electrically and mechanically isolated from the cathode.

These tubes are "pure triodes", meaning that there is no screen grid to be strapped to the plate in order to achieve triode operation. The classic triodes have very low plate resistance and low voltage gain. Many require significantly higher plate supply voltages than ordinary pentodes. In exchange for these limitations, these tubes offer very linear characteristic curves, making possible the design of low- distortion amplifiers that use little or no local or global feedback. The sound of a pure-triode amplifier is reputed to be exceedingly musical, with a natural harmonic structure, very low grain or noise, and a realistic, inviting nature. Triode adherents claim that the pure-triode output stage is sonically superior to one constructed with strapped screen grid pentodes. Other listeners will find the pure-triode amplifier to be colored, restricted in bandwidth, inefficient, and overpriced.

Single-ended triode amplifiers have been very popular in Japan for some time, and are making a limited comeback in North America.

Q: What is the difference between a single-ended and push-pull amplifier?

A: A push-pull output stage uses one or more pairs of output devices connected in a symmetrical arrangement such that output current flows to the load first through one half of the circuit and then through the other half. The advantages of the push-pull topology are higher efficiency, higher power output, much lower even-order distortions, immunity from power supply ripple, and zero DC current in the output transformer primary.

In contrast, the single-ended output stage employs only one set of output devices which conduct continuously throughout the output current cycle. This forces the stage to be operated in class A mode, limiting the available power output and greatly lowering efficiency. Total harmonic distortion is higher because there is no cancellation of even-order harmonics. Power supply ripple is not rejected by the single-ended output.

The most significant difficulty of the single-ended output stage is that the output transformer is required to carry a large DC current in its primary. Due to magnetic saturation and nonlinearity effects, a very special output transformer design is required. Such a transformer is large, heavy, expensive, and has a low power rating. The resulting amplifier is restricted in bandwidth at both extremes of the audio spectrum and produces a great deal of distortion. To minimize distortion (and to add to the single-ended mystique), it has become fashionable to design single-ended amplifiers with pure-triode output stages.

While no one claims the pure-triode, single-ended amplifier is "neutral" or "accurate", devotees of the genre describe in almost mystical terms the sonic attributes of these amplifiers. The word "magic" is often used. Listeners will have to judge for themselves.

Q: What are the meanings of Class A, B, and C?

A: Because virtually all active devices pass current in only one direction, it is necessary to go to some trouble in order to amplify audio signals, which are alternating currents. There are basically two strategies for making one-way components amplify two-way signals. The first is to use a pair (or pairs) of devices arranged so that one half of the circuit conducts current exclusively during positive swings of the signal, and the other half conducts exclusively during negative swings of the signal. This arrangement is called "push-pull" operation.

The other strategy is to superimpose a direct current on the AC signal of such a magnitude that the combined current remains net positive at the negative signal peaks. The result is that the amplifying device is never required to reverse the direction of its current flow. The superimposed direct current, which is known as a bias current, may be filtered out of the amplified signal using a transformer or blocking capacitor. In its simplest form, this type of circuit is known as "single-ended".

If we consider purely sinusoidal signals, it is clear that the output device in the single-ended circuit conducts current during 100% of the audio signal cycle. In contrast, each device in the push pull circuit conducts during exactly 50% of the signal cycle. It is also possible to construct amplifiers in which the output devices conduct for less than 50% of the signal cycle, although these circuits are generally not employed in audio amplifiers. The Class A, B, and C designations refer, respectively, to these three modes of operation.

Returning to the push-pull, Class B amplifier, it can be seen that during zero crossings of the output signal, the positive half of the circuit switches off just as the negative half begins to conduct, and vice versa. At the precise instant that the output current is zero, no current flows in either half of the circuit, i.e., there is no bias current. In practice, amplifying devices are quite nonlinear near their low-current cutoff points. A type of nonlinearity called "crossover distortion" can be eliminated if the Class B circuit is modified so that a modest bias current flows while the amplifier is idling. The small overlap in conduction between the two halves of the circuit smooths over the transition that occurs during zero crossings. Because the bias currents in the positive and negative halves of the circuit are are equal and opposite, they cancel one another automatically. This is why the net DC current in a push-pull output transformer primary is zero, and why solid-state push-pull amplifiers require no output DC blocking capacitors.

As the push-pull bias current increases, the conduction angle of each half of the circuit increases from the minimum value of 50%. If the bias current is set to a value equal to one half of the maximum output signal current, the conduction angle will equal 100% and the amplifier will be operating in Class A mode. The flow of bias current in the absence of signal current dissipates energy in the form of heat. Thus, the efficiency of the amplifier is reduced as the bias current increases from zero to full Class A operation.

An amplifier that is biased part-way between Class B and Class A operation is said to operate in Class AB mode. In vacuum tube amplifiers, an additional distinction between Class AB1 and Class AB2 is made. In Class AB1, the driver stage has a high output impedance and clips at the onset of output tube grid current flow. In Class AB2, the driver stage has a low output impedance, allowing it to drive the output grids linearly into the positive grid current region. This allows greater output power with a given bias current.

Despite the efficiency penalty, many listeners believe that Class A amplifiers drive difficult loads with more authority and a smoother sound than comparable class AB units.

--------------------------------------------------------------------------------
Henry A. Pasternack
Member Scientific Staff
Bell Northern Research, Montreal
E-mail: [email protected]
« Last Edit: Nov 10, 2003 at 09:22 AM by arnoldc »

Offline levi

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Re:Vacuum Tubes Overview
« Reply #26 on: Nov 10, 2003 at 09:59 PM »
Arnold,

 I merged your thread with an old thread about vacuum tubes by Vax. OK ito it will help other members understand more what is a tube.  :D

Wag lang masiraan ng ulo kakapost at kausapin ang sarili  ;D ;D ;D

levi

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Re:Vacuum Tubes Overview
« Reply #27 on: Nov 10, 2003 at 10:11 PM »
Arnold,

 I merged your thread with an old thread about vacuum tubes by Vax. OK ito it will help other members understand more what is a tube.  :D

Wag lang masiraan ng ulo kakapost at kausapin ang sarili  ;D ;D ;D

levi


levi...that's a good one...hahahaha!

sir arnoldc...thanks for this...very informative for tube starters like me... 8)

Fyne|EAR|Hana|Technics|SAEC|Wiim|Western Electric|Audiolab|Acrolink|Oyaide|Oppo|Tellurium

Offline arnoldc

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Re:Vacuum Tubes Overview
« Reply #28 on: Nov 11, 2003 at 09:15 AM »
levi, muchas gracias señor

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Re: Vacuum Tubes Overview
« Reply #29 on: May 24, 2004 at 01:45 PM »
Hi Fellow Members,

We would like to submit for your reference the specs of the Tube amp which we are reviewing for distribution :

Product Specification
·   Structure: Monoural Power Amplifier
·   Output Power: pure class A 28W
·   Circuit Form: CR coupled(pre), Transformer Drive
·   Used Tube: 6SL7GT, 6SN7GT, 300B, 845 each 1 tube
·   Input Terminal: Unballanced 1
·   Output Structure: 2 types speaker output(4/8 ohm) with gold plated and Bnana plug supported
·   Input Sensitivity: 0.8Vrms
·   Distortion: Less than 0.1% (1W/1kHz)
·   S/N Ratio: Over 90dB
·   Input Impedance: 100kohm
·   External Dimension: 300 x 420 x 180 mm   Weight: 33kg
·   All Toroidal Transformers Mounted

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